A curable composition capable of being cured by actinic rays such as ultraviolet rays or electron beams or heat has been applied to various fields, for example, coating materials for plastics, paper, or wood work; adhesives; printing inks, printed circuit boards, or electrical insulations.
In recent years, particularly a curable composition for printing inks, coating materials or adhesives is required to have higher weather resistance and higher adhesion to a substrate. As ink-jet ink employing such a curable composition, there is a UV-curable ink.
The UV-curable ink has been noted recently in respect to relatively low odor, rapid drying property, and capability of recording on a recording medium having no ink absorption. UV-curable ink jet inks are disclosed, for example, in Japanese Patent O.P.I. Publication No. 6-20204 and Japanese Patent Publication No 2000-504778. UV-curable inks are required which have a low viscosity, and provide an ink layer with high fastness, high flexibility, and high adhesion to a recording sheet. Curing speed of these UV-curable ink jet inks is likely to change due to kinds of recording sheets, or operating conditions.
An ink composition containing a radically polymerizable compound is susceptible to oxygen. Therefore, when such-ink composition is used in a small amount as ink droplets, curing is likely to be inhibited. Inks employing a cationically polymerizable compound, disclosed in Japanese Patent O.P.I. Publication Nos. 2001-220526, 2002-188025, 2002-317139, and 2003-55449), are not affected by oxygen, however, they have problem in that the polymerization reaction is susceptible to moisture in the molecular level (or humidity).